Multiple line telephone instruments using four conductors for the instrument, one pair for signaling and the other pair for speech transmission are known from art such as U.S. Pat. No. 3,935,396 issued Jan. 27, 1976 to J. Barsellotti, et al. In this patent, space division switching was employed to provide the switching control for bidirectional speed transfer. A custom coding arrangement was employed in time division multiplex over the data bus to provide the supervisory controls for analog speech transmission over the switched path. Systems using the principle of the patent have been made and sold by the International Telephone and Telegraph Corporation as systems of the MKS-100 Key System, and are described in an article entitled "Data Controlled Key Telephone System" by J. McNeilly and J. Barsellotti on pages 187-190 of Electrical Communication. Volume 52 Number 3.1977.
A series of patents issued to the Bell System cover multiple line telephone instruments using four data conductors and two analog speech conductors. For example, see U.S. Pat. No. 3,749,848 issued July 31, 1973 to D. Knollman, et al. A similar approach is shown in earlier U.S. Pat. No. 3,519,757 to H. Anderson, et al issued July 7, 1970 with a fifteen conductor data bus.
In these systems, the instrument is activated over the data bus, control and supervisory signals are transmitted over the data bus to control the switching or the analog speech pair for that instrument and to transmit further control signals bidirectionally over the data bus.
In the first-mentioned prior art system, U.S. Pat. No. 3,935,396, the signaling over the data pair or data bus was transmitted in diphase form with square wave signals of one frequency forming one binary digit and signals of a second frequency at twice the first frequency forming the other binary digit. Line condition data transmitted and data received at the instrument was in the form of an eight bit signal followed by an eight bit complementary check signal. Sixteen bits of data comprises a full signal.